Electronic displays are those electronic components that can convert electrical signals into visual images in real time that are otherwise suitable for direct interpretation—i.e. viewing—by a person. Such displays typically serve as the visual interface between persons and electronic devices such as computers, televisions, various forms of machinery, and numerous other applications.
The use of electronic displays has grown rapidly in recent years driven to some extent by the personal computer revolution, but also by other utilitarian and industrial applications in which such electronic displays have begun to partially or completely replace traditional methods of presenting information such as mechanical gauges, and printed paper.
One of the most familiar types of electronic display is the conventional television in which a cathode ray tube (CRT) produces the image. The nature and operation of cathode ray tubes has been well understood for several decades and will not be otherwise discussed in detail herein, except to highlight the recognition that the nature of a CRT's operation requires it to occupy a three-dimensional area that generally is directly proportional to the size of the CRT's display surface. Thus, in the conventional television set or personal computer, the CRT display tends to have a depth that is the same as, or in some cases greater than, the width and height of its display screen.
Accordingly, the desirability for an electronic display that can use space more efficiently has been well recognized for some time, and has driven the development of a number of various devices that are often referred to collectively as “flat-panel displays.” A number of techniques have been attempted, and some are relatively well developed, for flat-panel displays. These include gas discharge, plasma displays, electroluminescence, light emitting diodes (LEDS), cathodoluminescence, and liquid crystal displays (LCDs). To date, flat panel technologies have been generally widely used in certain portable displays and in numerical displays that use fewer (i.e. less than several hundred) characters. For example, the typical display on a hand-held calculator can be characterized as a flat-panel display even though it tends to operate in only one color, typically using either LEDs or LCDs.
Light emitting diodes have generally been recognized as likely candidate devices for flat panel displays for a number of reasons. These include their solid state operation, the ability to make them in relatively small sizes (thus potentially increasing resolution), and potentially a relatively low cost of manufacture. To date, however, flat panel displays incorporating LEDs have failed to reach their theoretical potential in the actual marketplace.
LED flat panel displays have lacked success in penetrating the technology and the marketplace for several reasons. One basic reason is the lack of suitable or commercial acceptable LEDs in the three primary colors (red, green and blue), that can be combined to form appropriate true color flat panel images. In that regard, color can be defined for certain purposes as “that aspect of visual sensation enabling a human observer to distinguish differences between two structure-free fields of light having the same size, shape and duration.” McGraw-Hill Encyclopedia of Science and Technology, 7th Edition, Volume 4, p. 150 (1992). Stated differently, color can be formed and perceived by the propagation of electromagnetic radiation in that portion of the electromagnetic spectrum that is generally referred to as “visible.” Typically, if the electromagnetic spectrum is considered to cover wavelengths from the long electrical oscillations (e.g. 1014 micrometers) to cosmic rays (10−9 micrometers), the visible portion of the spectrum is considered to fall from about 0.770 micrometers (770 nanometers “nm”) to about 0.390 micrometers (390 nm). Accordingly, to emit visible light of even a single color, a light emitting diode must produce radiation with a wavelength of between about 390 and 770 nm. In that regard, the theory and operation of light emitting diodes and related photonic devices in general are set forth in appropriate fashion in Sze, Physics of Semiconductor Devices, Second Edition, pp. 681-838 (1981) and will not otherwise be discussed in great detail herein, other than as necessary to describe the invention. A similar but more condensed discussion can be found in Dorf, The Electrical Engineering Handbook, pp. 1763-1772 (CRC Press 1983).
In order for a display of light emitting diodes to form combinations of colors, those diodes must emit primary colors that can be mixed to form other desired colors. A typical method for describing color is the well-recognized “CIE chromaticity diagram” which was developed several decades ago by the International Commission on Illumination (CIE), and a copy of which is reproduced herein as FIG. 6. The CIE chromaticity diagram shows the relationship among colors independent of brightness. Generally speaking, the colors visible to the human eye fall on the CIE chart within an area defined by a boundary. As FIG. 6 shows, the boundary is made up of a straight line between 380 and 660 nm, and a curved line which forms the remainder of the generally cone-shaped area.
Although the color perceptions of individual persons may of course differ, it is generally well understood and expected that colors visible by most persons fall within the boundaries of the CIE diagram.
Accordingly, the color output of electronic displays, including flat panel displays, can be plotted on the CIE diagram. More particularly, if the wavelengths of the red, green, and blue primary elements of the display are plotted on the CIE diagram, the color combinations that the device can produce are represented by the triangular area taken between the primary wavelengths produced. Thus, in FIG. 6, the best available devices are plotted as the lines between the wavelengths of about 655 or 660 nanometers for aluminum gallium arsenide (AlGaAs) red devices, about 560 nanometers for gallium phosphide green devices, and about 480 nanometers for silicon carbide (SiC) blue devices. Gallium phosphide can also be used in red-emitted devices, but these generally emit in the 700 nm range. Because the human eye is less responsive at 700 nm, the devices tend to lack brightness and thus are often limited to applications where maximum brightness is less critical. Similarly, silicon carbide blue devices have only been commercially available for approximately a decade. As the triangle formed by joining these wavelengths on the CIE diagram demonstrates, there exist entire ranges of colors in both the upper and lower portions of the CIE diagram that even these most recently available displays simply cannot produce by the limitations of the physics of their LEDs.
Stated somewhat more simply, although certain LED displays can be described as “full color,” they cannot be classified as “true color” unless and until they incorporate LEDs that are respectively more green, more red, and more blue, and that are formed from devices that can have sufficient brightness to make the devices worthwhile. For simplicity's sake, however, the terms “full color” and “true color” are used synonymously hereinafter.
In regard to color and brightness, and as set forth in the reference materials mentioned above, the characteristics of an LED depend primarily on the material from which it is made, including its characteristic as either a direct or indirect emitter. First, as noted above and as generally familiar to those in the electronic arts, because blue light is among the shortest wavelengths of the visible spectrum, it represents the highest energy photon as among the three primary colors. In turn, blue light can only be produced by materials with a bandgap sufficiently wide to permit a transition in electron volts that corresponds to such a higher energy shorter wavelength photon. Such materials are generally limited to silicon carbide, gallium nitride, certain other Group III nitrides, and diamond. For a number of reasons, all of these materials have been historically difficult to work with, generally because of their physical properties, their crystallography, and the difficulty in forming them into both bulk crystals and epitaxial layers, both of which are generally (although not exclusively) structural requirements for light emitting diodes.
As noted above, some SiC blue LEDs—i.e. those in which SiC forms the active layer—have become available in commercially meaningful quantities in recent years. Nevertheless, the photon emitted by SiC results from an “indirect” transition rather than a “direct” one (see Sze supra, §12.2.1 at pages 684-686). The net effect is that SiC LEDs are limited in brightness. Thus, although their recent availability represents a technological and commercial breakthrough, their limited brightness likewise limits some of their applicability to displays, particularly larger displays that are most desirably used in bright conditions; e.g. outdoor displays used in daylight.
Accordingly, more recent work has focused on Group III (Al, In, Ga) nitrides, which have bandgaps sufficient to produce blue light, and which are direct emitters and thus offer even greater brightness potential. Group III nitrides present their own set of problems and challenges. Nevertheless, recent advances have placed Group III nitride devices into the commercial realm, and a number of these are set forth in related patents and copending applications including U.S. Pat. No. 5,393,993 and Ser. No. 08/309,251 filed Sep. 20, 1994 for “Vertical Geometry Light Emitting Diode With Group II Nitride Active Layer and Extended Lifetime”; Ser. No. 08/309,247 filed Sep. 20, 1994 for “Low Strain Laser Structure With Group III Nitride Active Layers”; and Ser. No. 08/436,141 filed May 8, 1995 for “Double Heterojunction Light Emitting Diode With Gallium Nitride Active Layer”, the contents of each of which are incorporated entirely herein by reference.
As another disadvantage, flat panel displays in the current art are generally only “flat” in comparison to CRTs, and in reality have some substantial thickness. For example, a typical “flat” LED display is made up of a plurality of LED lamps. As used herein, the term “lamp” refers to one or more light emitting diodes encased in some optical medium such as a transparent polymer, and with an appropriate size and shape to enhance the perceived output of the LED. In turn, the lamps must be connected to various driving circuits, typically a multiplexing circuit that drives rows and columns in a two-dimensional matrix of such devices. These in turn require appropriate power supplies and related circuitry. The net result are devices that—although thin compared to CRTs—do have significant physical depth.
For example, LED flat panel displays of any size are typically always several inches in depth and few if any are produced that are less than an inch in depth in actual use. Indeed, some of the largest flat panel displays with which the public might be familiar (i.e. stadium scoreboards and the like) use either enough LEDs or incandescent lamps to require significant heat transfer capabilities. For example, a stadium-size flat display is typically backed by an atmospherically controlled space; i.e. an air conditioned room; to take care of the heat that is generated.
Accordingly, the need exists and remains for a flat panel display formed of light emitting diodes that can produce a full range of colors rather than simply multiple colors, and which can do so in a truly thin physical space.